Electrophotography and electrostatic printing are well known image reproduction methods as is the reproduction of color images by these methods, for instance, in color copiers, color printers and color proofers.
In electrophotographic color copiers, the color original is projected onto a photoconductor sequentially through several color separation filters, such as blue, green and red. These filters separate the yellow, magenta and cyan color components of the original, respectively. Such color components then are sequentially toned on the photoconductor with toners of appropriate colors. The toner deposits then are sequentially transferred to a receptor to reproduce the original. In many instances, a black color also is used to enhance contrast.
The main difference between electrophotographic color printers and color copiers is that in color printers, the color original is scanned, the separated color components are electronically processed, digitized and stored if so desired. Such electronic data then is used to expose the photoconductor by means of a laser scanner.
In electrophotographic color proofing, the photoconductor is exposed sequentially to color separation films and toned sequentially with toners of appropriate colors. The color toner deposits then are transferred sequentially to a receptor, such as printing stock paper. Such pre-press color proofing processes are disclosed, for example, in U.S. Pat. Nos. 3,809,555 and 3,862,848. An apparatus for the production of electrophotographic pre-press proofs is described, for example, in U.S. Pat. Nos. 4,556,309 and 4,557,583.
It is known that for high image quality in electrophotographic color reproduction processes it is necessary to use fine particle size liquid toners. In certain such processes, for instance in color proofing, it is essential to have very uniformly filled in solid image areas. Additionally, the density of the image areas must be within narrow tolerances as specifically required over each proof and from proof to proof.
With liquid toners, however, it is necessary to operate at slow toning speeds in order to obtain uniform fill-in of solid images without any appreciable variation in the required image density. Variations in fill-in and density, at least in part, are caused by liquid toners being very sensitive to minor surface voltage or charge density variations on the photoconductor. These variations are common and result in differences in the rate of toner attraction.
Slow toning appears to compensate for such differences or variations, provided they are not more than about 5 to 7 percent of the nominal surface voltage. Slow toning, however, slows down the throughput and, for economic reasons, there is an obvious need for fast throughput.
The method of the invention provides virtually instantaneous liquid toning whereby complete fill-in of solid image areas at any required density can be obtained. Such toning virtually provides no variations in image density which can be caused by surface voltage variations on the photoconductor, even where such voltage variations can be 10 or even 20 percent of the nominal surface voltage. The method of the invention particularly is applicable to color proofers, color copiers, color printers and the like, henceforth referred to in general as color printers. The method, however, also is applicable to black and white or single color processes.